The present invention relates, in general, to an infrared detector and, more particularly, to a cooled infrared detector to be cooled at a low temperature for operation.
An infrared detector or an infrared sensor is used widely in diverse fields for various purposes, such as meteorological survey by an artificial satellite, safeguarding against crimes, preventing disasters, geological survey, resource exploration and medical infrared thermography, for its capability of detecting the presence, shape, temperature, composition and the like of matters without touching the matters. lnfrared detectors are classified roughly into thermal infrared detectors, and photoelectric infrared detectors employing semiconductors.
The sensitivity of the thermal infrared detector, in general, is not dependent on wavelength. However, the thermal infrared detector is unsuitable for real-time detection because the sensitivity and response speed of the thermal infrared detector are not high enough for real-time detection. On the other hand, the photoelectric infrared detector has high sensitivity and high response speed, however, the element of the photoelectric infrared detector must be cooled to temperature approximately equal to that of liquid nitrogen. Photoelectric infrared detectors are classified into photoconductive infrared detectors, photovoltaic infrared detectors and MIS infrared detectors. The photoconductive infrared detector detects electromagnetic radiation by the variation of the resistance of a photoconductive element. A known photoconductive infrared detector employs a photoconductive element formed of a compound semiconductor crystal, such as HgCdTe.
Such a photoconductive element is cooled nearly to the temperature of liquid nitrogen to secure high sensitivity by cooling a detecting unit provided with the photoconductive element and contained in a highly heat-insulating vacuum vessel by a refrigerant, such as liquid nitrogen, or by a cryogenic refrigerator.
An example of a conventional infrared detector will be described with reference to FIG. 1. The infrared detector has a vacuum vessel 10 comprising an outer cylinder 14 formed of, for example, an alloyed metal such as that sold under the trademark "Kovar", and an inner cylinder 16 formed of glass and having an outer surface coated with a gold film 15 deposited by evaporation. The space between the outer cylinder 14 and the inner cylinder 16 is evacuated. The outer cylinder 14 and the inner cylinder 16 are mounted on a base 18 formed of Kovar. The base 18 is secured to a support member 20 connected to a helium dilution refrigerator 12. An annular ceramic plate 22 attached to the outer cylinder 14 is connected to lead wires 25 and 27 to transfer a detection signal provided by the infrared sensing element 24 to an external device. An infrared sensing element 24 of a multielement type formed of a compound semiconductor, such as HgCdTe, is attached adhesively to the upper surface of the upper wall of the inner cylinder 16. A germanium window 14a is formed in the upper wall of the outer cylinder 14 to receive infrared rays therethrough. The germanium window 14a serves as a band-pass filter that transmits only infrared rays of frequencies in a predetermined frequency band.
When the helium dilution refrigerator 12 is operated, the infrared sensing element 24 is cooled nearly to the temperature of liquid nitrogen by means of a rod 28 formed of a stainless steel and a heat-conductive spring 28 formed of a copper alloy to detect infrared rays 30 indicated by alternate long and short dash lines.
Another conventional infrared detector similar in construction to that shown in FIG. 1 and having an inner cylinder and an infrared sensing element, which are similar respectively to the inner cylinder 16 and the infrared sensing element 24 of FIG. 1, is provided with a cold shield on the inner cylinder. The cold shield serves as a band-pass filter that transmits only infrared rays of frequencies in a predetermined frequency band.
In the conventional infrared detector shown in FIG. 1, some of the infrared rays 30 transmitted through the germanium window 14a fall on the infrared sensing element 24 after being reflected by the gold film 15 formed over the outer surface of the inner cylinder 16, and the inner surface of the outer cylinder 14 as indicated by arrows Y. The reflected infrared rays, namely, stray infrared rays, are a phantom input signal added to the infrared rays 30 received by the infrared sensing element 24 from the object of detection, namely, signal rays, to cause faulty detection.
Furthermore, the germanium window 14a serving as a band-pass filter transmits only the incident infrared rays of frequencies in a limited frequency band. Therefore, the infrared detector is unable to detect infrared rays of a plurality of different frequency bands even if the infrared detector is provided on its inner cylinder with a plurality of infrared sensing elements respectively having different detecting frequency bands.
Accordingly, a plurality of infrared detectors provided respectively with infrared transmitting windows differing from each other in transmission frequency band must be used for detecting infrared rays in a plurality of different frequency bands.